Readjusting devices of the afore-described type are needed in brake calipers intended to perform, in addition to the hydraulic function of the operating brake, the function of a mechanical parking brake. The brake pad wear is compensated by a readjusting device as the operating distance of the mechanical parking brake would otherwise increase by the amount of the pad wear. During operation of such a readjusting device, the brake must be prevented from automatically readjusting by an excessive amount, i.e., the operating clearance between brake pad and brake disc must be maintained. The pad compression and the caliper expansion at elevated hydraulic pressures also must be compensated. Moreover, the readjusting device must be insensitive to high hydraulic pressure and pressure fluctuations, temperature influences, and vibration and must insure operating safety throughout an extended period of time.
To prevent the readjusting device from readjusting at a high brake pressure and, hence, at a resilient deformation of the caliper and of the pad, measures have already been suggested (see German Patents No. 38 00 734, No. 38 00 735, and No. 38 03 564) which prevent adjustment above a predetermined brake pressure. Devices of this type are of a relatively complex design and require a very careful setting of the components involved in switching off the readjusting device. These prior art readjusting devices are limited in that pad wear resulting from a high brake pressure be readjusted during the braking operation.
German Patent No. 26 15 404 teaches switching off the readjusting device at a relatively low pressure and, subsequently, at a decreasing pressure, readjusting the distance no longer covered by the piston because the pad has worn. The disadvantage of the state-of-the-art readjusting device is that it requires a cylinder of a substantially complex design composed of a plurality of individual components. The friction cone of the rotating readjusting device undercuts a part of the cylinder so that the cylinder must be substantially dissembled for repairing or replacing individual components. This problem applies especially to the replacement of the sealant required for initiating the pressure control.
U.S. Pat. No. 3,920,103 displaceably arranges the rotatable readjusting element within the piston in a spring-loaded manner. A sealant is provided on the rotatable element to seal the pad-sided piston cavity against the remaining space of the brake cylinder. However, the readjusting device described therein is of a design such that the spring loading of the rotatable readjusting member tends to keep the friction cone disengaged permanently, resulting, with respect to the hand brake, in comparatively extended readjusting distances.